Chemically sensitive field effect transistors and uses thereof in electronic nose devices

ABSTRACT

A system having an electronic device. The electronic device has an array of chemically sensitive sensors. The sensors detect volatile organic compounds and have field effect transistors. The transistors have non-oxidized, functionalized silicon nanowires. The nanowires have surface Si atoms. The device has a plurality of functional groups that form a direct Si—C bond with the silicon nanowires, wherein Si is a surface Si atom and C is a carbon atom of the functional group. The functional groups are selected from the group consisting of: alkyl, cycloalkyl, alkenyl, alkynyl, aryl, heterocyclyl, heteroaryl, alkylaryl, alkylalkenyl, alkylalkynyl, alkylcycloalkyl, alkylheterocyclyl and alkylheteroaryl groups, and derivatives thereof, wherein said functional groups are other than methyl and 1-butyl. The plurality of functional groups are attached to 50-100% of the surface Si atoms.

FIELD OF THE INVENTION

The present invention relates to an electronic device for detectingvolatile organic compounds with high sensitivity. In particular, thepresent invention provides chemically sensitive field effect transistorsof non-oxidized, functionalized silicon nanowires and methods of usethereof.

BACKGROUND OF THE INVENTION

Electronic nose devices perform odor detection through the use of anarray of cross-reactive sensors in conjunction with pattern recognitionmethods. In contrast to the “lock-and-key” model, each sensor in theelectronic nose device is widely responsive to a variety of odorants. Inthis architecture, each analyte produces a distinct signature from thearray of broadly cross-reactive sensors. This configuration allows toconsiderably widen the variety of compounds to which a given matrix issensitive, to increase the degree of component identification and, inspecific cases, to perform an analysis of individual components incomplex multi-component mixtures. Pattern recognition algorithms canthen be applied to the entire set of signals, obtained simultaneouslyfrom all the sensors in the array, in order to acquire information onthe identity, properties and concentration of the vapor exposed to thesensor array. Various algorithms and computer controlled systems forolfactometry known in the art are disclosed for example in U.S. Pat.Nos. 6,411,905, 6,606,566, 6,609,068, 6,620,109, 6,767,732, 6,820,012,and 6,839,636, among others.

Micro-organisms produce patterns of volatile organic compounds (VOCs)that are affected by the type and age of culture media. Patterns of VOCscan also be used as biomarkers of various diseases, e.g., acute asthma,uremia, cirrhosis, cystinuria, trimethylaminuria, etc. These diseasebiomarkers can be found in the bodily fluids of a patient, including inthe serum, urea, and breath. Characteristic VOCs display differentpatterns at different stages of the disease.

Various devices and methods for VOC detection and analysis aredisclosed, for instance, in U.S. Pat. Nos. 6,319,724, 6,411,905,6,467,333, 6,606,566, 6,609,068, 6,620,109, 6,703,241, 6,767,732,6,820,012, 6,839,636, 6,841,391, and in U.S. Pat. Appl. No.2001/0041366. A transition metal oxide gas sensor is disclosed anddescribed in U.S. Pat. No. 6,173,602.

Excluding a few individual instances, the detection levels of thesedevices are in the range of 1-100 parts per million (ppm). In order todetect VOCs with higher sensitivity, pre-concentrating the vapors to bedetected prior to measurement is required. Consequently, real-timemeasurement of minute quantities of VOCs remains a challenge.

The use of Gas-Chromatography (GC), GC-lined Mass-Spectroscopy (GC-MS),Quartz Crystal Microbalance (QCM) as well as other comparable techniquesfor analysis of volatile biomarkers indicative of certain diseases, isimpeded by several factors. These factors include the need for expensiveequipment, the degree of expertise required to operate such instruments,the length of time required to obtain data acquisition, and othertechnical problems in sampling, data analysis, etc. Mostly, the GC-MStechnique is limited to the ppm level of concentrations, while manydisease biomarkers are present at concentration levels of less than onepart per billion (ppb).

Similar to olfactory receptors, increased sensitivity as well as on/offrates of chemical sensors is typically achieved by reducing thedimensions of the sensing apparatus. Chemical sensors made ofnanomaterials are more sensitive, more controlled, and more suitable todifferentiate between subtle differences in mixtures of volatilebiomarkers. Silicon nanowires (Si NWs) offer unique opportunities forsignal transduction associated with selective recognition of biologicalor chemical species of interest.

Oxide-coated silicon nanowire field effect transistors (Si NW FETs) havebeen modified with amino siloxane functional groups to impart highsensitivity towards pH (Patolsky and Lieber, Mater. Today, 2005, 8:20-28). The Si NW field effect transistors were further modified with avariety of biological receptors to selectively detect biological speciesin solution. Oxide-coating of a Si NW is believed to induce trap statesat the Si/Si-oxide interface thus acting as a dielectric layer. This inturn lowers and consequently limits the effect of gate voltage on thetransconductance of Si NW field effect transistors. This limitationaffects the response of sensors based on oxide-coated Si NW field effecttransistors to their environment. In a typical SiO₂-coated Si NW fieldeffect transistor, the transconductance responds weakly to the appliedgate voltage, V_(g), where conductivity changes by two orders ofmagnitude between V_(g)=−5V and V_(g)=+5 V, with no significant on/offstate transition within this gate-bias region. This behavior iscompatible with the characteristics of oxidized Si wherein both theSi/SiO₂ interface and the SiO₂ surface defects trap and scattercarriers, and as a result, decrease the effect of V_(g) (Lupke, SurfSci. Rep., 1999, 35:75-161). On the contrary, devices that are based onnon-oxidized Si NWs as well as those based on macroscopic planar Si(111) surfaces, exhibit low interface state density. Yet, non-oxidizedSi NWs as well as Si surfaces that are terminated with hydrogen tend toundergo oxidation upon exposure to ambient conditions, resulting in theformation of defects in the sensors.

It has been reported by the inventor of the present invention, that SiNWs modified by covalent binding to a methyl functional group, showatmospheric stability, high conductance values, and less surfacedefects. These methyl functionalized Si NWs were shown to formair-stable Si NW field effect transistors having on-off ratios in excessof 10⁵ over a relatively small (±2 V) gate voltage swing (Haick et al.,J. Am. Chem. Soc., 2006, 128: 8990-8991). However, exposure of thesemethyl-functionalized devices to analytes barely provides sensingresponses, most probably due to the low ability of the methyl groups toadsorb vapor/liquid analytes. Further modifications of the methylfunctional groups for sensing applications at minute concentration downto the ppb levels, are not feasible.

Hence, there is an unmet need for a highly sensitive reliable device toanalyze mixtures of volatile organic compounds. Furthermore, there is anunmet need for an inexpensive, efficient, convenient, reliable, andportable device to analyze mixtures of volatile biomarkers.

SUMMARY OF THE INVENTION

The present invention provides an electronic device for detectingvolatile organic compounds (VOCs), which is more sensitive than knownsystems serving a similar purpose. The device disclosed herein,comprises field effect transistors of non-oxidized functionalizedsilicon nanowires (Si NWs). The present invention further relates to asystem comprising an electronic device comprising an array of chemicallysensitive sensors in conjunction with learning and pattern recognitionalgorithms. The learning and pattern recognition algorithms receivesensor output signals which are analyzed using methods such asartificial neural networks and principal component analysis and aresubsequently compared to stored data. Methods of preparing said devicesand methods of use thereof for detecting and quantifying specificcompounds are disclosed as well.

The invention is based in part on the unexpected finding thatnon-oxidized silicon nanowire-based sensors provide improved sensingcapabilities. The lack of oxide layer on the surface of the nanowiresimproves the sensitivity of the detectors thus providing the detectionof minute quantities of volatile organic compounds. The detection ofminute quantities of volatile organic compounds enables theidentification of biomarkers from body secretions without the need forpre-concentration.

According to a first aspect, the present invention provides anelectronic device comprising at least one chemically sensitive sensorfor the detection of volatile organic compounds, wherein the chemicallysensitive sensor comprises field effect transistors comprisingnon-oxidized, functionalized silicon nanowires (Si NWs), wherein thefunctional group is other than methyl.

According to another aspect, the present invention provides a systemcomprising an electronic device for detecting volatile organiccompounds, wherein said electronic device comprises an array ofchemically sensitive sensors comprising field effect transistors ofnon-oxidized, functionalized silicon nanowires (Si NWs), and learningand pattern recognition analyzer wherein said learning and patternrecognition analyzer receives sensor signal outputs and compares them tostored data.

In one embodiment, the electronic devices of the present inventiondetect volatile organic compounds with sensitivity below one part permillion (ppm). In another embodiment, the electronic devices detectvolatile organic compounds with sensitivity of less than 100 parts perbillion (ppb). In yet another embodiment, the electronic devicesdisclosed herein detect volatile organic compounds with sensitivity ofone part per billion (ppb) or less.

In some embodiments, the Si NW field effect transistors are manufacturedin a top-down approach. In alternative embodiments, the Si NW fieldeffect transistors are manufactured in a bottom-up approach.

According to certain embodiments, the surface of the Si NWs is modifiedwith a plurality of compounds via a direct Si—C bond. In currentlypreferred embodiments, these compounds form film layers on the surfaceof the nanowires, without intervening oxide layers. For example, thepresent invention uses compounds that attach through Si—C—C, Si—C═C, andSi—C≡C bonds.

In other embodiments, the functional groups which are used to modify thesurface of the nanowires include, but are not limited to: alkyl,cycloalkyl, alkenyl, alkynyl, aryl, heterocyclyl, heteroaryl, alkylaryl,alkylalkenyl, alkylalkynyl, alkylcycloalkyl, alkylheterocyclyl andalkylheteroaryl groups; combinations and derivatives thereof. Thefunctional groups can be substituted by one or more halogens selectedfrom the group consisting of fluorine, chlorine, bromine, and iodine.Additional substituents are haloalkyl, acyl, amido, ester, cyano, nitro,and azido.

In certain embodiments, the functional groups used to modify the surfaceof the nanowires include, but are not limited to, ethyl, isopropyl,tert-butyl, hexyl, octyl, and phenyl, cyclic C₆ hydrocarbonyl,1,3-cyclohexadienyl, 1,4-cyclohexadienyl, cyclohexyl, H-terminatedbicyclo[2.2.2]octyl, Cl-terminated bicyclo[2.2.2]octyl, and combinationsthereof.

In other embodiments, the functional groups used to modify the surfaceof the nanowires include, but are not limited to, 1-pentyl, 1-dodecyl,2-hexynyl, 1-octenyl, 1-pentenyl, 1-dodecenyl, 1-octadecenyl,cis-2-pentenyl, trans-2-hexenyl, 2,3-dimethyl-2-pentenyl, styrenyl andfive-, six-, eight-membered ring derivatives thereof; and combinationsthereof.

In yet other embodiments, the functional groups used to modify thesurface of the nanowires include, but are not limited to,phenylacetylenyl, 1-phenyl-2-(trimethylsilyl) acetylenyl, 1-octynyl,dodec-1-ynyl, 1-trimethylsilyldodec-1-ynyl, pentynyl,diphenylphosphino-acetylenyl, arynyl, diphenyl-phosphinoethynyl, andcombinations thereof.

In certain embodiments, the surface of the nanowires is modified withultra thin polymer films of e.g. polypropylene or polynorbornene. In oneembodiment, these polymer films are grown via layer-by-layer orring-opening metathesis polymerization approaches. According tocurrently preferred embodiments, the polymer films have thicknessesranging from about 1 nm to about 500 nm.

The present invention also encompasses an electronic device designatedfor the detection of volatile organic compounds, comprising an array ofchemically sensitive sensors. The array of sensors comprises a pluralityof sensors between 2 to 1000 sensors, more preferably between 2 to 500sensors, even more preferably between 2 to 250 sensors, and mostpreferably between 2 to 125 sensors in an array.

According to another aspect, the present invention further provides asystem comprising an electronic device designated for the detection ofvolatile organic compounds, comprising an array of chemically sensitivesensors in conjunction with a learning and pattern recognition analyzer,wherein said learning and pattern recognition analyzer receives sensoroutput signals and compares them to stored data. The learning andpattern recognition analyzer may utilize various algorithms includingalgorithms based on artificial neural networks, multi-layer perception(MLP), generalized regression neural network (GRNN), fuzzy inferencesystems (FIS), self-organizing map (SOM), radial bias function (RBF),genetic algorithms (GAS), neuro-fuzzy systems (NFS), adaptive resonancetheory (ART) and statistical methods such as principal componentanalysis (PCA), partial least squares (PLS), multiple linear regression(MLR), principal component regression (PCR), discriminant functionanalysis (DFA) including linear discriminant analysis (LDA), clusteranalysis including nearest neighbor, and the like.

According to another aspect, the present invention provides a method fordetermining at least one of the composition and concentration ofvolatile organic compounds in a sample using the electronic devices ofthe present invention, comprising the steps of: (a) providing a systemcomprising an electronic device for detecting volatile compoundscomprising an array of chemically sensitive sensors of non-oxidized,functionalized silicon nanowire field effect transistors, and a learningand pattern recognition analyzer, wherein said learning and patternrecognition analyzer receives sensor output signals from the electronicdevice and compares them to stored data, (b) exposing the sensor arrayof said electronic device to the sample, and (c) using patternrecognition algorithms to detect the presence of said volatile compoundsin the sample.

According to another aspect, the present invention provides a method fordiagnosing a disease in a subject, comprising: (a) providing a systemcomprising an electronic device for detecting volatile organic compoundscomprising an array of chemically sensitive sensors of non-oxidized,functionalized silicon nanowire field effect transistors, and a learningand pattern recognition analyzer, wherein said learning and patternrecognition analyzer receives sensor output signals and compares them tostored data, (b) exposing the sensor array of said electronic device tothe breath of a subject, or to the headspace of a container in which abodily fluid of the subject has been deposited, and (c) using patternrecognition algorithms to detect volatile organic compounds, in thesample indicative of a disease in said subject.

In yet another aspect, the present invention provides the use of anelectronic device comprising an array of chemically sensitive sensorswherein the chemically sensitive sensors comprise field effecttransistors comprising non-oxidized, functionalized silicon nanowires,and a learning and pattern recognition analyzer, wherein said learningand pattern recognition analyzer receives sensor output signals andcompares them to stored data, for the preparation of an apparatus fordetecting volatile organic compounds. In a currently preferredembodiment, the use disclosed herein is designated towards detectingvolatile organic compounds that are indicative of a disease in asubject.

The present invention further provides a system for diagnosing a diseasein a subject comprising exposing an electronic device comprising anarray of chemically sensitive sensors wherein the chemically sensitivesensors comprise field effect transistors comprising non-oxidized,functionalized silicon nanowires to the breath of a subject, or to theheadspace of a container in which a bodily fluid of the subject has beendeposited, and using pattern recognition algorithms to receive sensoroutput signals and compare them to stored data.

Bodily fluids or secretions that can be tested by this method include,but are not limited to, serum, urine, feces, sweat, vaginal discharge,saliva and sperm. Many diseases or disorders can be diagnosed by themethods and systems of the invention, including, but not limited to,oral infections, periodontal diseases, halitosis, ketosis, yeastinfections, pneumonia, lung infections, cancer, sexually transmitteddiseases, vaginitis, nephritis, bilirubin production, renal disease,cardiovascular disease, hypercholesterolemia, gastrointestinalinfections, diabetes, and phenylketonuria. According to a preferredembodiment, the present invention provides a method of diagnosingcancer.

Further embodiments and the full scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter. However, it should be understood that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a Si NW field effect transistorarrangement used for chemical sensing without a reference electrode. Themolecular layer is directly bonded to the semiconductor and the gatingis done from the back. ‘A’ represents analyte molecules, and ‘S’represents sensing molecules.

FIG. 2 is a schematic diagram illustrating the differentiation betweenodorants using an array of broadly-cross reactive sensors, in which eachindividual sensor responds to a variety of odorants, in conjugation withpattern recognition algorithms to allow classification. ‘A’—rawmeasurements, ‘B’—normalized measurements, ‘C’—feature vector, ‘D’—odorclass (confidence level), ‘E’—post processed odor class, ‘F’—decisionmaking, ‘G’—classification, ‘H’—dimensionality reduction, and ‘I’—signalpreprocessing.

FIG. 3 is a Scanning Electron Micrograph of “as-grown” Si NWs.

FIG. 4 is a high resolution Transmission Electron Micrograph of anindividual Si NW coated with a thin native oxide layer.

FIG. 5(A-B) are high resolution-Scanning Electron Micrographs of Si NWsbefore (5A), and after (5B) alkylation for 24 hours. The Si NWs areabout 2-4 μm in length and about 50 nm in diameter.

FIG. 6 is a pseudo-first-order-fitting of the kinetics of formation offunctionalized Si NWs.

FIG. 7(A-I) are XPS data of C 1 s region, showing C—Si (284.1±0.1 eV),C—C (285.2±0.1 eV) and C—O (286.7±0.1 eV) peaks of freshly-preparedsamples of non-oxidized Si NWs functionalized with (7A) methyl, (7B)ethyl, (7C) propyl, (7D) butyl, (7E) pentyl, (7F) hexyl, (7G) octyl,(7H) decyl, and (7I) undecyl.

FIG. 8(A-H) are XPS data of Si 2p region (8A, 8C, 8E, and 8G) and C isregion (8B, 8D, 8F, and 8H). The non-oxidized Si NWs were functionalizedwith the following functional groups: (8A and 8B) CH₃—Si, (8C and 8D)CH₃—CH₂—CH₂—Si, (8E and 8F) CH₃—CH═CH—Si, and (8G and 8H) CH₃—C≡C—Si.

FIG. 9 is the observed oxidation (SiO₂/Si_(2P) peak ratio) at differentexposure times to ambient conditions.

FIG. 10 is a Scanning Electron Micrograph of a Si NW device having fourAluminum (Al) contacts.

FIG. 11(A-B) are graphs of the responses of the Si NW field effecttransistor devices to hexane vapor exposure at low concentrations. (11A)Non-oxidized Si NWs functionalized with CH₃ (CH₃—Si—NW; upper curve) orbutyl (Butyl-Si—NW; lower curve) functional groups; (11B) bareSiO₂-coated Si NWs (upper curve), SiO₂-coated Si NWs modified with CH₃(CH₃—SiO₂—Si NW; middle curve), or SiO₂-coated Si NWs modified withbutyl (Butyl-SiO₂—Si NW; lower curve) functional groups.

FIG. 12 is a principal components plot of an array of six Si NW fieldeffect transistors upon exposure to simulated “healthy (H)” and“cancerous (C)” breath samples.

FIG. 13 is a schematic diagram showing the Sensor Array connected to theLearning and Pattern Recognition Analyzer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an electronic device for detectingvolatile organic compounds at levels as low as 100 parts per billion(ppb) or less. The electronic device comprises chemically sensitivesensors comprising field effect transistors of non-oxidized,functionalized silicon nanowires. The invention further provides asystem comprising an array of sensors and pattern recognition analyzerwhich uses algorithms, such as principal component analysis and neuralnetwork algorithms, to classify and detect a wide variety of volatileorganic compounds. Further provided are methods of use thereof indetecting biomarkers indicative of certain medical disorders.

According to the principles of the present invention, the electronicdevices disclosed herein comprise chemically sensitive field effecttransistors (FETs) of non-oxidized, functionalized silicon nanowires (SiNWs; FIG. 1). Sensing is obtained through adsorption of vapors toprovide changes in electrical resistance. The electrical signals arethen conveyed to a pattern recognition analyzer to generate qualitativeidentification and preferably quantitative analysis of desired volatileorganic compounds (VOCs). A schematic diagram of the differentiationbetween odorants using the electronic nose devices is illustrated inFIG. 2. The array of sensors is exposed to a variety of VOCs to providean electronic response vs. time (2^(nd) box on the left). Thedimensionality is then reduced wherein the data is represented by a newbasis set (f₂ vs. f₁; 3^(rd) box on the left). This representationallows to classify the different odors (1, 2 & 3; 4^(th) box on theleft). The procedure can be iteratively performed until satisfactoryodor classification is achieved.

The present invention discloses for the first time, Si NW surfacesmodified with Si—C bonds, wherein a significantly better coverage of theSi NWs with alkyl functional groups excluding methyl is obtained.Specifically, functionalization of the Si NWs with C₂-C₁₁ alkyl chainsusing a versatile two step chlorination/alkylation process, producescoverage in the range of 50-100% of the Si NW surface sites. This isapproximately 1.5 times the coverage obtained for equivalent 2D Si (100)surfaces. The higher coverage provides Si NW surfaces having improvedsurface passivation and increased stability against oxidation. Thealkylated Si NW surfaces of the present invention show high chemicalstability at ambient conditions, as compared to alkylated 2D Sisubstrates.

Control over the surface chemistry of Si NWs is particularly importantfor the electrical performance of sensors composed of Si NW field effecttransistors. The Si NWs of the present invention possess superiorelectrical properties in comparison to fully or partially oxidized SiNWs. These functionalized Si NWs can thus be used to fabricateelectronic devices such as, but not limited to, Si-basedphotoelectrochemical cells with improved energy conversion. Additionaluse of oxide-free surfaces is for radial epitaxy on the nanowires torealize vertical P—N junctions for solar cells, or radial Si/Gesuperlattices for application in optoelectronics.

Device

The electronic device described in the present invention usesfinely-tuned arrays of surface-modified, non-oxidized Si NW field effecttransistor-based sensors. The nanowires are approximately 5-120 nm indiameter, having a cylinder-like shape with a circle-like cross section,or equivalent dimensions wherein the nanowires have other crosssectional shapes including, but not limited to, trapezoidal, triangular,square, or rectangular. Si NWs having diameters (or equivalentdimensions for shapes other than cylinder) larger than 120 nm possesselectrical/physical properties similar to planar Si. Si NWs withdiameters (or equivalent dimensions for shapes other than cylinder) lessthan 5 nm consist mostly of SiO₂, with very low percentage of Si core.Thus, the Si NWs whose dimensions exceed the 5-120 nm range, are lesssuitable for sensing applications in accordance with the presentinvention.

Without being bound by any theory or mechanism of action, elimination ofthe intervening oxide layer from the Si NW field effect transistorsprovides increased sensitivity to the analytes to be detected. Thechemical modification thus provides stable Si—C bonds even upon exposureto air and/or humidity, and further endows the Si NWs with chemicalinertness and good electronic properties possibly due to the passivationof Si NW surface states. The modifications of the Si NW surfaces can betailor-made to control the electrical properties of the Si NWs by, forexample, utilizing adsorptive molecular dipoles on the Si NW surface,applying back gate voltage, and/or use of four-probe configuration. Themodification further allows the control over the contact resistancebetween the Si NWs thus enabling the elimination of the electrodes,further providing the required sensitivity for detecting cancerbiomarkers as well as other volatile organic compounds.

Formation of the Si NW Field Effect Transistors. The non-oxidized Si NWfield effect transistor-based sensors of the present invention can bemanufactured in two alternative manners: a bottom-up approach or atop-down approach.

In one embodiment of the invention, sensors of Si NW field effecttransistors are manufactured through a bottom-up approach. Si NWs thatare grown by, for example, vapor-liquid-solids, chemical vapordeposition (CVD), or oxide-assisted growth, are dispersed from organicsolvent (e.g., isopropanol or ethanol) onto a doped Si substratecontaining a thin film of dielectric layer (e.g., SiO₂, ZrO₂, etc.). Thedeposited Si NWs can be “bare” or “as-synthesized” ones, namely, withoxide layer and/or without being modified by organic molecules, oralternatively the deposited Si NWs can be non-oxidized and furtherpossess various functionalities. The source/drain contacts to the Si NWsare introduced by either one of these techniques: electron beamlithography followed by evaporation of a metal that forms an ohmiccontact, focused ion beam (FIB), and contact printing. The devices arethen annealed to improve the quality of the contacts.

In another embodiment, the sensors are manufactured through a top-downapproach. The fabrication process initiates from a SOI-SIMOX wafer, withthin top silicon layer, insulated from the silicon substrate by a buriedsilicon dioxide layer. Mask definition is performed by high resolutione-beam lithography. A bilayer PMMA resist is used. The exposure isperformed using e-beam lithography with an acceleration voltage of 30kV. The resist is then developed in a solution of MiBK:IPA 1:3. Thepattern is transferred from the PMMA to the top SiO₂ layer by BHF etch.The central region, where the silicon is defined, is linked throughsmall connections to the device leads. A 35 wt % KOH solution, saturatedwith isopropyl alcohol (IPA), is used. The nanowire then forms in thecentral region.

Surface modification of the Si NW Field Effect Transistors.Functionalizing the nanowires, whether before or after integration inthe field effect transistor device, is performed through the use ofreagents having different backbones and functional groups. Desiredreagents are synthesized and attached to the Si NW surfaces, via Si—Cbonds. The functional groups used include, but are not limited to,alkyl, cycloalkyl, alkenyl, alkynyl, aryl, heterocyclyl, heteroaryl,alkylaryl, alkylalkenyl, alkylalkynyl, alkylcycloalkyl,alkylheterocyclyl and alkylheteroaryl groups; combinations andderivatives thereof. The functional groups can be substituted by one ormore halogens selected from the group consisting of fluorine, chlorine,bromine, and iodine. Other substitutions within the scope of the presentinvention include functionalization with haloalkyl, acyl, amido, ester,cyano, nitro, and azido groups.

An “alkyl” group refers to a saturated aliphatic hydrocarbon, includingstraight-chain, branched-chain and cyclic alkyl groups. In oneembodiment, the alkyl is other than methyl. In another embodiment, thealkyl group has 2-12 carbons designated here as C₂-C₁₂-alkyl. In anotherembodiment, the alkyl group has 2-6 carbons designated here asC₂-C₆-alkyl. In another embodiment, the alkyl group has 2-4 carbonsdesignated here as C₂-C₄-alkyl. The alkyl group may be unsubstituted orsubstituted by one or more groups selected from halogen, haloalkyl,acyl, amido, ester, cyano, nitro, and azido.

A “cycloalkyl” group refers to a non-aromatic mono- or multicyclic ringsystem. In one embodiment, the cycloalkyl group has 3-10 carbon atoms.In another embodiment, the cycloalkyl group has 5-10 carbon atoms.Exemplary monocyclic cycloalkyl groups include cyclopentyl, cyclohexyl,cycloheptyl and the like. An alkylcycloalkyl is an alkyl group asdefined herein bonded to a cycloalkyl group as defined herein. Thecycloalkyl group can be unsubstituted or substituted with any one ormore of the substituents defined above for alkyl.

An “alkenyl” group refers to an aliphatic hydrocarbon group containingat least one carbon-carbon double bond including straight-chain,branched-chain and cyclic alkenyl groups. In one embodiment, the alkenylgroup has 2-8 carbon atoms (a C₂₋₈ alkenyl). In another embodiment, thealkenyl group has 2-4 carbon atoms in the chain (a C₂₋₄ alkenyl).Exemplary alkenyl groups include ethenyl, propenyl, n-butenyl,i-butenyl, 3-methylbut-2-enyl, n-pentenyl, heptenyl, octenyl,cyclohexyl-butenyl and decenyl. An alkylalkenyl is an alkyl group asdefined herein bonded to an alkenyl group as defined herein. The alkenylgroup can be unsubstituted or substituted through available carbon atomswith one or more groups defined hereinabove for alkyl.

An “alkynyl” group refers to an aliphatic hydrocarbon group containingat least one carbon-carbon triple bond including straight-chain andbranched-chain. In one embodiment, the alkynyl group has 2-8 carbonatoms in the chain (a C₂₋₈ alkynyl). In another embodiment, the alkynylgroup has 2-4 carbon atoms in the chain (a C₂₋₄ alkynyl). Exemplaryalkynyl groups include ethynyl, propynyl, n-butynyl, 2-butynyl,3-methylbutynyl, n-pentynyl, heptynyl, octynyl and decynyl. Analkylalkynyl is an alkyl group as defined herein bonded to an alkynylgroup as defined herein. The alkynyl group can be unsubstituted orsubstituted through available carbon atoms with one or more groupsdefined hereinabove for alkyl.

An “aryl” group refers to an aromatic monocyclic or multicyclic ringsystem. In one embodiment, the aryl group has 6-10 carbon atoms. Thearyl is optionally substituted with at least one “ring systemsubstituents” and combinations thereof as defined herein. Exemplary arylgroups include phenyl or naphthyl. An alkylaryl is an alkyl group asdefined herein bonded to an aryl group as defined herein. The aryl groupcan be unsubstituted or substituted through available carbon atoms withone or more groups defined hereinabove for alkyl.

A “heteroaryl” group refers to a heteroaromatic system containing atleast one heteroatom ring wherein the atom is selected from nitrogen,sulfur and oxygen. The heteroaryl contains 5 or more ring atoms. Theheteroaryl group can be monocyclic, bicyclic, tricyclic and the like.Also included in this definition are the benzoheterocyclic rings.Non-limiting examples of heteroaryls include thienyl, benzothienyl,1-naphthothienyl, thianthrenyl, furyl, benzofuryl, pyrrolyl, imidazolyl,pyrazolyl, pyridyl, pyrazinyl, pyrimidinyl, pyridazinyl, indolyl,isoindolyl, indazolyl, purinyl, isoquinolyl, quinolyl, naphthyridinyl,quinoxalinyl, quinazolinyl, cinnolinyl, pteridinyl, carbolinyl,thiazolyl, oxazolyl, isothiazolyl, isoxazolyl and the like. Theheteroaryl group can be unsubstituted or substituted through availableatoms with one or more groups defined hereinabove for alkyl.

A “heterocyclic ring” or “heterocyclyl” group refers to a five-memberedto eight-membered rings that have 1 to 4 hetero atoms, such as oxygen,sulfur and/or in particular nitrogen. These five-membered toeight-membered rings can be saturated, fully unsaturated or partiallyunsaturated, with fully saturated rings being preferred. Preferredheterocyclic rings include piperidinyl, pyrrolidinyl pyrrolinyl,pyrazolinyl, pyrazolidinyl, morpholinyl, thiomorpholinyl, pyranyl,thiopyranyl, piperazinyl, indolinyl, dihydropyranyl, tetrahydrofuranyl,dihydrothiophenyl, tetrahydrothiophenyl, dihydropyranyl,tetrahydropyranyl, and the like. An alkylheterocyclyl is an alkyl groupas defined herein bonded to a heterocyclyl group as defined herein. Theheterocyclyl group can be unsubstituted or substituted through availableatoms with one or more groups defined hereinabove for alkyl.

“Ring system substituents” refer to substituents attached to aromatic ornon-aromatic ring systems including, but not limited to, H, halo,haloalkyl, C₈)alkyl, (C₂-C₈)alkenyl, (C₂-C₈)alkynyl, (C₅-C₁₀)aryl, acyl,amido, ester, cyano, nitro, azido, and the like.

The term “halogen” or “halo” as used herein alone or as part of anothergroup refers to chlorine, bromine, fluorine, and iodine. The term“haloalkyl” refers to an alkyl group having some or all of the hydrogensindependently replaced by a halogen group including, but not limited to,trichloromethyl, tribromomethyl, trifluoromethyl, triiodomethyl,difluoromethyl, chlorodifluoromethyl, pentafluoroethyl,1,1-difluoroethyl bromomethyl, chloromethyl, fluoromethyl, iodomethyl,and the like.

The term “acyl” as used herein encompasses groups such as, but notlimited to, formyl, acetyl, propionyl, butyryl, pentanoyl, pivaloyl,hexanoyl, heptanoyl, octanoyl, nonanoyl, decanoyl, undecanoyl,dodecanoyl, benzoyl and the like. Currently preferred acyl groups areacetyl and benzoyl.

In particular, exemplary functional groups include, but are not limitedto,

(a) Methyl, ethyl, isopropyl, tert-butyl, hexyl, octyl, phenyl, cyclicC₆ hydrocarbonyl, 1,3-cyclohexadienyl, 1,4-cyclohexadienyl,cyclohexenyl, and H- and Cl-terminated bicyclo[2.2.2]octanyl. Thesefunctional groups bind to the Si surface through a Si—C—C bond. Withoutbeing bound by any theory or mechanism of action, the functionalizationof the silicon nanowires with saturated non-oxidized functional groupssuch as alkyl and cycloalkyl is expected to provide high sensitivitytowards biomarkers that adsorb between the chains of the molecular film.

(b) 1-pentynyl, 1-dodecynyl, 2-hexynyl, 1-octenyl, 1-pentenyl,1-dodecenyl, 1-octadecenyl, cis-2-pentenyl, trans-2-hexenyl,2,3-dimethyl-2-pentenyl, styrenyl and five-, six-, and eight-memberedring derivatives thereof. These functional groups bind to the Si surfacethrough a Si—C═C bond, hence are expected to increase charge transferthrough the attached backbones. Without being bound by any theory ormechanism of action, these functional groups are expected to provideincreased sensitivity towards biomarkers that adsorb on the surface ofthe molecular film.

(c) phenylacetylenyl, 1-phenyl-2-(trimethylsilyl) acetylenyl, 1-octynyl,dodec-1-ynyl, 1-trimethylsilyldodec-1-ynyl, pentynyl,diphenylphosphino-acetylenyl, arynyl, and diphenyl-phosphinoethynyl.These functional groups bind to the Si surface through a Si—C≡C bridge,hence are expected to increase the charge transfer through the attachedbackbones. Additionally, these molecules equilibrate between the energylevels of the Si core interfaces and the energy levels of the molecularfrontier orbitals effectively. Without being bound by any theory ormechanism of action, this provides another mechanism by which moleculescan interact with the biomarkers and produce targeted sensitivityenhancements, especially for species which are adsorbed on top of theorganic phase.

(d) ultra thin polymer films of e.g. polypropylene or polynorbornene.The attachment of polymeric chains to the Si NW surface can be performedvia ruthenium ring-opening metathesis polymerization catalyst. Thismethod allows to better control the thickness of the polymer that isattached to the silicon substrate. Currently preferable thicknessesrange from about 0.9 nm to about 550 nm. Without being bound by anytheory or mechanism of action, adjusting the thickness of the adsorbinglayer to an optimal value is expected to increase the absorption of thetargeted biomarkers, and, thus to enhance the sensitivity of thefabricated device.

Functionalization of the Si NW field effect transistors can be performedby several procedures, non-limiting examples of which will now bedescribed.

Functionalization through Chlorination Route. Chlorinated Si (111)surfaces can be prepared by two different methods. In one chlorinationmethod, an H-terminated sample is immersed into a saturated solutionincluding PCl₅, PBr₅, and PI₅ that contains a few grains of radicalinitiator, such as C₆H₅OOC₆H₅. The reaction solution is heated to90-100° C. for 45 minutes. In another chlorination method, anH-terminated sample is placed into a Schlenk reaction tube andtransported to a vacuum line. Approximately 50-200 Torr of Cl_(2(g)) isintroduced through the vacuum line into the reaction tube, and thesample is illuminated for 30 seconds with a 366 nm ultraviolet light.Excess Cl_(2(g)) is then removed under vacuum, and the flask istransported to the N_(2(g))-purged flush box. The chlorine-terminated Sisurfaces are molecularly modified by immersion in 1.0-3.0 molar R—MgX,where R signifies the backbone of molecules, and X=Cl, Br, or I. Thereaction is performed for 1.5-16 hours at 70-80° C. wherein longer andbulkier molecular chains require longer reaction times. Excess THF, orother pertinent organic solvent, is added to all reaction solutions forsolvent replacement. When the reaction ends, the samples are removedfrom the reaction solution and then rinsed in THF, CH₃OH, andoccasionally TCE. Samples are then sonicated for about 5 minutes inCH₃OH and CH₃CN and dried.

Functionalization by Lewis Acid-Mediated Terminal Alkene Reduction.Freshly etched, H-terminated Si (111) surfaces are functionalized byimmersion in approximately equal volumes of the molecule of interest and1.0 M C₂H₅AlCl₂ in hexane at room temperature for 12 hours. Samples areremoved from solution and rinsed in THF, CH₂Cl₂, and CH₃OHconsecutively, and then dried.

Functionalization by Electrochemical Reduction of R—MgI. Samples aremounted to a cell for surface functionalization reactions. Samples arethen etched by filling the cell with 40% NH₄F_((aq)). After 20 minutes,the etching solution is removed and the cell is filled with H₂O to rinsethe sample surface. The H₂O is then removed from the cell, and thesample is dried under a stream of N_(2(g)). The cell is then moved intothe N_(2(g))-purged flush box for electrochemical modification. Eachchamber of the electrochemical cell contains a section of Cu gauze thatserves as a counter electrode. A single counter electrode is introducedinto the solution. Molecular modification is performed using 3.0 MCH₃MgI in diethyl ether by applying 0.1 mA·cm⁻² of constant anodiccurrent density for 5 minutes with continuous stirring of the solution.After surface modification, the cell is rinsed with CH₂Cl₂ and CH₃OH,consecutively. The cell is then dismantled, and the top and bottom ohmiccontacts are scribed off to leave behind only the portion of the waferthat had been exposed to the reaction solution. This wafer is rinsedfurther in CH₃OH, sonicated in CH₃OH, further sonicated in CH₃CN, anddried with a stream of N_(2(g)).

Other modifying agents include ultra-thin monomer or polymer films, suchas polypropylene or polynorbornene. Attachment of the polymers mentionedherein to the Si NW surface can be done via ruthenium ring-openingmetathesis polymerization catalyst. In this manner, control over thethickness of the polymer attached to the silicon substrate fromsub-nanometers to hundreds of nanometers is achieved.

Analysis

According to one embodiment, a method to determine the composition andconcentration of volatile organic compounds (VOCs) in a sample,comprising exposure of the sensors of the electronic device to thesample and using pattern recognition algorithms in order to identify andpossibly quantify desired VOCs in a given sample, is provided in thepresent invention. Thus, the electronic device of the present inventionfurther includes a learning and pattern recognition analyzer. Inpractice, the analyzer receives signal outputs or patterns from thedevice and analyses them by various pattern recognition algorithms toproduce an output signature. By comparing an unknown signature with adatabase of stored or known signatures, volatile organic compounds canbe identified.

Algorithms for sample analysis, suitable for identifying and possiblyquantifying VOCs include, but are not limited to, principal componentanalysis, Fischer linear analysis, neural network algorithms, geneticalgorithms, fuzzy logic, pattern recognition, and the like. Afteranalysis is completed, the resulting information can, for example, bedisplayed on display, transmitted to a host computer, or stored on astorage, device for subsequent retrieval.

Many of the algorithms are neural network based algorithms. A neuralnetwork has an input layer, processing layers and an output layer. Theinformation in a neural network is distributed throughout the processinglayers. The processing layers are made up of nodes that simulate theneurons by the interconnection to their nodes.

In operation, when a neural network is combined with a sensor array, thesensor data is propagated through the networks. In this manner, a seriesof vector matrix multiplications are performed and unknown analytes canbe readily identified and determined. The neural network is trained bycorrecting the false or undesired outputs from a given input. Similar tostatistical analysis revealing underlying patterns in a collection ofdata, neural networks locate consistent patterns in a collection ofdata, based on predetermined criteria.

Suitable pattern recognition algorithms include, but are not limited to,principal component analysis (PCA), Fisher linear discriminant analysis(FLDA), soft independent modeling of class analogy (SIMCA), K-nearestneighbors (KNN), neural networks, genetic algorithms, fuzzy logic, andother pattern recognition algorithms. In some embodiments, the Fisherlinear discriminant analysis (FLDA) and canonical discriminant analysis(CDA) as well as combinations thereof are used to compare the outputsignature and the available data from the database.

In other embodiments, principal component analysis is used. Principalcomponent analysis (PCA) involves a mathematical technique thattransforms a number of correlated variables into a smaller number ofuncorrelated variables. The smaller number of uncorrelated variables isknown as principal components. The first principal component oreigenvector accounts for as much of the variability in the data aspossible, and each succeeding component accounts for as much of theremaining variability as possible. The main objective of PCA is toreduce the dimensionality of the data set and to identify new underlyingvariables.

In practice, principal component analysis compares the structure of twoor more covariance matrices in a hierarchical fashion. For instance, onematrix might be identical to another except that each element of thematrix is multiplied by a single constant. The matrices are thusproportional to one another. More particularly, the matrices shareidentical eigenvectors (or principal components), but their eigenvaluesdiffer by a constant. Another relationship between matrices is that theyshare principal components in common, but their eigenvalues differ. Themathematical technique used in principal component analysis is calledeigenanalysis. The eigenvector associated with the largest eigenvaluehas the same direction as the first principal component The eigenvectorassociated with the second largest eigenvalue determines the directionof the second principal component. The sum of the eigenvalues equals thetrace of the square matrix and the maximum number of eigenvectors equalsthe number of rows of this matrix.

Applications

The present invention provides a method to detect volatile compounds ina sample, comprising exposing the sensors of the electronic nose deviceto a sample and using pattern recognition algorithms in order toidentify and possibly quantify the components of the sample.

In one embodiment, the present invention is used to detect minuteconcentrations of volatile organic compounds. In a currently preferredembodiment, the electronic devices of the present invention providedetection of volatile organic compounds at levels as low as parts perbillion (ppb) or less.

According to one embodiment, the Si NW sensors are in a field effecttransistor configuration. These field effect transistors are typicallyused for sensing chemical processes, and are thus known as CHEMFETs.There are many different varieties of CHEMFETS, most of which are basedon a common principle, namely the presence of molecules or ions affectthe potential of the conducting field effect transistor channel eitherby directly influencing the gate potential (e.g., for a catalyticallyactive metal gate) or by changing the potential distribution between a“reference electrode gate” and the semiconductor. Since infinitesimalchemical perturbations can result in large electrical response, Si NWsensors are sensitive to, and can be used to detect, minuteconcentrations of chemicals. Without being bound by any theory ormechanism of action, the Si NW sensors used along with a reference gateand an ideal polar layer, induce a significant field in the channel.This field ensues due to the overall potential difference between theground and reference electrodes. Thus, the field is induced tocompensate for the potential drop.

According to other embodiments, chemical sensing devices can be producedusing Si NW field effect transistors with no reference electrode. Suchdevices have generally been referred to as molecularly controlledsemiconductor resistors (MOCSERs). In MOCSERs, the traditional gatingelectrode is either present at the back, with a molecular layer adsorbeddirectly on the semiconductor, or is replaced altogether by a molecularlayer adsorbed on a (typically ultra-thin) dielectric. Without beingbound by any theory or mechanism of action, in either one of saidconfigurations, binding of molecules from the gas or liquid phase to the“chemical sensing molecules”, possibly changes the potential in theconducting channel. Consequently, the current between source and drainis modified and the device serves as a sensor. Such devices possess highchemical sensitivity.

In one embodiment, the present invention is used to diagnose a diseasein a subject, by detecting biomarkers indicative of the disease in theheadspace of a container of a bodily fluid, such as, but not limited to,serum, urine, feces, vaginal discharge, sperm, saliva etc. The systemcan detect volatile organic compounds in breath that is directly exhaledby the subject towards the device, without a need for samplepre-concentration. Other possibilities include exhaling into a balloonand then exposing the collected breath to the electronic nose device.

In a preferred embodiment, the method described herein is used todiagnose cancer. GC-MS studies have shown that volatile C₄-C₂₀ alkanesand certain monomethylated alkanes and benzene derivatives appear to beelevated in instances of cancer. The compounds of interest are generallyfound at 1-20 ppb in healthy human breath, but can be seen indistinctive mixture compositions at elevated levels from 10-100 ppb inthe breath of diseased patients. The levels of volatile organiccompounds are elevated even at the early stages of the disease sincethey reflect a change in the human body chemistry. Also, biomarkers of aspecific disease (e.g., lung cancer) have distinctive mixturecompositions/patterns in comparison to other diseases (e.g., breastcancer).

In one embodiment, the present invention relates to the diagnosis ofcancer using the electronic nose device disclosed herein. The termcancer refers to a disorder in which a population of cells has become,in varying degrees, unresponsive to the control mechanisms that normallygovern proliferation and differentiation. Cancer refers to various typesof malignant neoplasms and tumors, including metastasis to differentsites. Non-limiting examples of cancers which can be detected by theelectronic devices of the present invention are brain, ovarian, colon,prostate, kidney, bladder, breast, lung, oral, and skin cancers.Specific examples of cancers are: adenocarcinoma, adrenal gland tumor,ameloblastoma, anaplastic tumor, anaplastic carcinoma of the thyroidcell, angiofibroma, angioma, angiosarcoma, apudoma, argentaffinoma,arrhenoblastoma, ascites tumor cell, ascitic tumor, astroblastoma,astrocytoma, ataxia-telangiectasia, atrial myxoma, basal cell carcinoma,benign tumor, bone cancer, bone tumor, brainstem glioma, brain tumor,breast cancer, vaginal tumor, Burkitt's lymphoma, carcinoma, cerebellarastrocytoma, cervical cancer, cherry angioma, cholangiocarcinoma, acholangioma, chondroblastoma, chondroma, chondrosarcoma, chorioblastoma,choriocarcinoma, larynx cancer, colon cancer, common acute lymphoblasticleukaemia, craniopharyngioma, cystocarcinoma, cystofibroma, cystoma,cytoma, ductal carcinoma in situ, ductal papilloma, dysgerminoma,encephaloma, endometrial carcinoma, endothelioma, ependymoma,epithelioma, erythroleukaemia, Ewing's sarcoma, extra nodal lymphoma,feline sarcoma, fibroadenoma, fibrosarcoma, follicular cancer of thethyroid, ganglioglioma, gastrinoma, glioblastoma multiforme, glioma,gonadoblastoma, haemangioblastoma, haemangioendothelioblastoma,haemangioendothelioma, haemangiopericytoma, haematolymphangioma,haemocytoblastoma, haemocytoma, hairy cell leukaemia, hamartoma,hepatocarcinoma, hepatocellular carcinoma, hepatoma, histoma, Hodgkin'sdisease, hypernephroma, infiltrating cancer, infiltrating ductal cellcarcinoma, insulinoma, juvenile angiofibroma, Kaposi sarcoma, kidneytumour, large cell lymphoma, leukemia, chronic leukemia, acute leukemia,lipoma, liver cancer, liver metastases, Lucke carcinoma, lymphadenoma,lymphangioma, lymphocytic leukaemia, lymphocytic lymphoma, lymphocytoma,lymphoedema, lymphoma, lung cancer, malignant mesothelioma, malignantteratoma, mastocytoma, medulloblastoma, melanoma, meningioma,mesothelioma, metastatic cancer, Morton's neuroma, multiple myeloma,myeloblastoma, myeloid leukemia, myelolipoma, myeloma, myoblastoma,myxoma, nasopharyngeal carcinoma, nephroblastoma, neuroblastoma,neurofibroma, neurofibromatosis, neuroglioma, neuroma, non-Hodgkin'slymphoma, oligodendroglioma, optic glioma, osteochondroma, osteogenicsarcoma, osteosarcoma, ovarian cancer, Paget's disease of the nipple,pancoast tumor, pancreatic cancer, phaeochromocytoma, pheochromocytoma,plasmacytoma, primary brain tumor, progonoma, prolactinoma, renal cellcarcinoma, retinoblastoma, rhabdomyo sarcoma, rhabdosarcoma, solidtumor, sarcoma, secondary tumor, seminoma, skin cancer, small cellcarcinoma, squamous cell carcinoma, strawberry haemangioma, T-celllymphoma, teratoma, testicular cancer, thymoma, trophoblastic tumor,tumourigenic, vestibular schwannoma, Wilm's tumor, or a combinationthereof.

The system of the present invention can further help diagnose othermedical disorders including, but not limited to, acute asthma, hepaticcoma, rheumatoid arthritis, schizophrenia, ketosis, cardiopulmonarydisease, uremia, diabetes mellitus, dysgeusia/dysosmia, cystinuria,cirrhosis, histidinemia, tyrosinemia, halitosis, and phenylketonuria.

The present invention also relates to non-oxidized functionalized Si NWsensors in which the functional group is tailor-made to allow forspecific identification of compounds selected from vapors of volatileorganic compounds. The technology of the present invention provides finetuning of the devices through modifying the functional groups attachedto the Si NW to high density functionalities which allow bettersignal/noise ratios.

Due to the miniaturized dimensions of the electronic nose device (in therange of 10-100 nanometers to a few micrometers), these devices could beinstalled in many electronic apparatuses. For example, these devicescould be integrated into a watch or cellular phone, to provide a warningsystem for the initiation of an infection or other disease in the bodyof an individual.

The system of the present invention can be used in many other differentapplications wherein the detection of volatile organic compounds isfeasible. These applications include, but are not limited to,environmental toxicology and remediation, medicine, materials qualitycontrol, food and agricultural products monitoring, heavy industrialmanufacturing (automotive, aircraft, etc.), such as ambient airmonitoring, worker protection, emissions control, and product qualitytesting; oil/gas petrochemical applications, such as combustible gasdetection, H₂S monitoring, hazardous leak detection and identification;hazardous spill identification, enclosed space surveying, utility andpower applications, such as emissions monitoring and transformer faultdetection; food/beverage/agriculture applications, such as freshnessdetection, fruit ripening control, fermentation process monitoring andcontrol, flavor composition and identification, product quality andidentification, and refrigerant and fumigant detection.

Additional applications include, but are not limited to,cosmetic/perfume applications, such as fragrance formulation, productquality testing, and fingerprinting; chemical/plastics/pharmaceuticalsapplications, such as fugitive emission identification, leak detection,solvent recovery effectiveness, perimeter monitoring, and productquality testing; hazardous waste site applications, such as fugitiveemission detection and identification, leak detection andidentification, transportation applications, such as hazardous spillmonitoring, refueling operations, shipping container inspection, anddiesel/gasoline/aviation fuel identification; building/residentialapplications, such as natural gas detection, formaldehyde detection,smoke detection, automatic ventilation control (cooking, smoking, etc.),and air intake monitoring; hospital/medical applications, such asanesthesia and sterilization gas detection, infectious diseasedetection, breath, wound and bodily fluids analysis, and telesurgery.

The principles of the present invention are demonstrated by means of thefollowing non-limitative examples.

EXAMPLES Example 1 Synthesis of the Silicon Nanowires (Si NWs)

Si NWs were prepared by the vapor-liquid-solid (VLS) growth method usingchemical vapor deposition (CVD) with silane on Si(111) substrates. Sisubstrates were etched in diluted HF to remove the native oxidefollowing by sputtering of a 2 nm thick Au film on the substrate. Thesample was transferred into the CVD chamber, and annealed at ˜580° C.with a pressure of ˜5×10⁻⁷ mbar for 10 minutes. The temperature was thendropped to ˜520° C. and a mixture of 5-10 sccm Ar and 5 sccm SiH₄ wasintroduced for 20 minutes at a pressure of 0.5-2 mbar to obtainedundoped Si NWs. FIG. 3 shows a typical Scanning Electron Micrograph ofSi NWs grown from gold (Au) seeds. FIG. 4 shows Transmission ElectronMicrograph of the Si NWs in which the majority of the NWs exhibit smooth50±10 nm diameter Si cores coated with 3-4 nm SiO₂, having lengths inthe range of 2-4 μm.

Doped Si NWs were prepared by the vapor-liquid-solid (VLS) growthtechnique under gas ratios of 10 sccm He, 5 sccm SiH₄, and 0.02 sccmB₂H₆ (2% in He), yielding p-type Si NWs doped with Boron. TEMcharacterization indicated that these NWs are essentially smooth havinga diameter of 52±8 nm. The surface of the Si NW was covered with nativeoxide and minute amounts of gold.

Example 2 Alkylation of Si Nanowires (Si NWs) through Si—C bond

Functionalization of the Si NWs of the present invention was performedusing a two-step chlorination/alkylation route. Prior to any chemicaltreatment, each sample was cleaned using a nitrogen (N_(2(g))) flow.Hydrogen-terminated Si NWs were then prepared by etching the amorphousSiO₂ coating. This was done through exposing the Si NWs to buffered HFsolution (pH=5) for 60 seconds followed by exposure to NH₄F for 30seconds. It is noteworthy that longer exposures to HF and/or NH₄Fresults in fluorination of the sample thus interfering with thealkylation process. The sample was then removed and rinsed in water for<10 seconds per each side to limit oxidation, and dried in N_(2(g))flowfor 10 seconds. The sample was transferred into a glove-box withN_(2(g))-atmosphere for functionalization.

Functionalization was preformed by immersing the sample into a saturatedsolution of PCl₅ in C₆H₅Cl (0.65M) that contained a few grains ofC₆H₅OOC₆H₅ to act as a radical initiator (Hassler and Koell, J.Organometal. Chem. 1995, 487, 223). The reaction solution was heated to90-100° C. for 5 minutes. The sample was then removed from the reactionsolution and rinsed in tetrahydrofuran (THF) followed by a methanol(CH₃OH) rinse and drying under a stream of N_(2(g)). Additionally,several samples were further rinsed with 1,1,1-trichloroethane (TCE)before drying under N_(2(g)) flow. The chlorine-terminated Si NWs werealkylated by immersion in 0.5M alkyl Grignard in THF (RMgCl: where Rrepresents an alkyl chain with 1-7 carbon atoms). The reaction wasperformed for 30-250 minutes at 80° C. Excess THF was added to allreaction solutions for solvent replacement. At the end of the reaction,the sample was removed from the reaction solution and was then rinsed inTHF, methanol, and occasionally TCE. The sample was then dried under astream of N_(2(g)). Though the PCl₅ is known to extremely damage andbreak the Si NWs in exposure of 10 minutes or more, High resolutionScanning Electron Micrographs (HRSEM, Zeiss Leo 982, Germany; operatedat 4 KV) confirmed that the alkylation process used herein did notdamage or break the Si NWs which remained with the same dimensions asprior to the alkylation (FIGS. 5A-B).

Example 3 The Kinetics of Formation of Functionalized Si NWs

The coverage of Si NW surfaces with various alkyl chains was plotted asa function of time. FIG. 6 shows a semi-logarithmic plot from which thepseudo-first-order rate constant of the reaction was calculated. Twodistinct regions are observed in the curves: at short time intervals anaccelerated rate is obtained (corresponds to a Kp1 slope) and at longertime intervals mildly increased rate is obtained (corresponds to a Kp2slope). Without being bound by any theory or mechanism of action, theincreased rate is attributed to the lack of steric hindrance betweenadjacent alkyl chains. This result is compatible with the kinetics of apseudo-first-order reaction.

The kinetics of pseudo first order (ln [n]=−K_(p1)*t) is detected for upto about 90% of a full coverage of the Si NWs followed by a decrease inrate thereafter. The two main regions in the alkylation process of theNWs are from zero to Γsat, and from Fsat to longer alkylation times(mostly 24 hours). Over 90% of the Si NW surfaces were covered with thealkyl molecules after short immersion time. Additionally, Kp1 was foundto decrease as the alkyl chain increases, namely the longer themolecular chains the longer the time required for alkylation. Incontrast, Kp2 was found essentially unaffected by the length of thealkyl chain indicating a “zero-order” reaction. Table 1 summarizes thesaturation level of the adsorption curve (Γsat values) for differentalkyl functional groups along with the related(C_(Si)/Si)_(alkyl)/(C_(Si)/Si)_(methyl) ratios. The ethylated surfacesshowed a (C_(Si)/Si)_(ethyl)/(C_(Si)/Si)_(methyl) value of 70±5%,indicating that ethyl (C₂) groups can be packed at a very high densitywithout major steric hindrance effects. Propyl (C₃), butyl (C₄), pentyl(C₅), hexyl (C₆), octyl (C₈), decyl (C₁₀) and undecyl (C₁₁) produced56±5%, 49±5%, 50±10%, 56±6%, 54±5%, 77±04%, and 57±02% coverage,respectively. The percentages of coverage are substantially higher thanthe coverage of the same functional groups on 2D Si (100) surfaces(Table 1). Furthermore, the time required to achieve maximum coverage ofthe molecules on Si NWs (where the molecules cover 50-100% of the atopSi sites) is 4 to 30 times shorter than that required for the 2Dsurfaces (where the molecules cover <55% of the atop Si sites). Withoutbeing bound by any theory or mechanism of action, these differencescould be attributed to surface energy, activation energy and sterichindrance effects between the adsorbed molecules wherein an increase inthe length of the alkyl chain increases the van der Waals diameterto >4.5-5.0 Å, significantly larger than the inter-nuclear distancebetween adjacent Si atoms (3.8 Å). In other words, Kp1 decreases as thealkyl chain length increases. For example, Kp1 of methyl group(2.64×10⁻²) is 38 times larger than Kp1 of decyl group (7.0×10⁻⁴). Anadditional factor which influences the decay in the rate constants isthe accessibility of the Si atoms to nucleophilic carbon attack. Thus,the curvature of the Si NWs reduces the steric hindrance effect betweenthe molecules allowing a higher surface coverage and a shorteralkylation time. Therefore, decreasing the diameter of the Si NWs isexpected to allow full-passivation of longer alkyl chains (>C₆).

Without being bound by any theory or mechanism of action, at highcoverage, a significant fraction of the available surface sites aresurrounded by occupied sites and cannot be accessed by a propagatingrandom walk, At this stage, the inter-steric-effect is more dominantthan the nucleophilic attack effect and the kinetic behavior is nolonger controlled by the nucleophile concentration thus switching to“zero-order” kinetics.

TABLE 1 Summary of the XPS results for C₁-C₁₁ chains bonded to Si NW and2D Si surfaces via Si—C bond Max. Max. C_(Si)/Si_(2p) coverage^((b))C_(Si)/Si_(2p) coverage^((b)) Γ_(sat) ^((a)) ratio on ratio for on 2DAlkyl [min] for Si NW Si NW Si 2D(100) Si (100) Methyl (C₁)  20 ± 20.135 ± 0.001 — 0.135 ± 0.001 — Ethyl (C₂)  50 ± 10 0.093 ± 0.003 70 ±5%  0.090 ± 0.02  60 ± 20% Propyl (C₃)  60 ± 10 0.075 ± 0.006 56 ± 5% 0.048 ± 0.002 35 ± 2%  Butyl (C₄)  65 ± 10 0.066 ± 0.004 49 ± 5%  0.049± 0.006 35 ± 4%  Pentyl (C₅)  90 ± 10 0.068 ± 0.012 50 ± 10% 0.051 ±0.003 35 ± 5%  Hexyl (C₆)  120 ± 10 0.076 ± 0.009 56 ± 6%  0.055 ± 0.00440 ± 4%  Octyl (C₈)  450 ± 20 0.073 ± 0.002 54 ± 5%  0.056 ± 0.002 40 ±10% Decyl (C₁₀) 1000 ± 50 0.104 ± 0.003 77 ± 04% 0.058 ± 0.012 40 ± 10%Undecyl 1000 ± 50 0.077 ± 0.003 57 ± 02% 0.029 ± 0.006 20 ± 6%  (C₁₁)^((a))Γsat is the time required to achieve 92 ± 3% of the saturationlevel of the adsorption curve. ^((b))Coverage is calculated as(C_(Si)/Si)_(alkyl)/(C_(Si)/Si)_(methyl).

Example 4 Characterization of the Functionalized Si NWs

Transmission electron microscopy (TEM) images of freshly-preparedfunctionalized Si NW samples showed a core diameter similar to theSiO₂-coated Si NWs. X-ray Photoelectron Spectroscopy (XPS) data from thecarbon 1 s (C 1s) emission region of the alkyl-functionalized Si NWs,was fitted to three peaks, namely C—Si at 284.1±0.1 eV, C—C at 285.2±0.1eV, and C—O at 286.7±0.1 eV. FIGS. 7A-7I show representative C 1 sregion XPS data and fits of non-oxidized Si NWs functionalized with (A)methyl at 20 minutes alkylation, (B) ethyl at 80 minutes alkylation, (C)propyl at 120 minutes alkylation, (D) butyl at 160 minutes alkylation,(E) pentyl at 160 minutes alkylation, (F) hexyl at 240 minutesalkylation, (G) octyl at 450 minutes alkylation, (H) decyl at 1000minutes alkylation, and (I) undecyl at 1000 minutes alkylation. Thepeaks were typically adjusted to produce fits that minimized thedifference between the full widths at half-maximum (FWHM). Peak centerswere allowed to float, while the center-to-center distances were fixedat 1.1 eV between the C—Si and the C—C emissions, and at 2.6 eV betweenthe C—O and the C—Si emissions. The integrated area under each carbonpeak was normalized to the integrated area under the silicon 2p (Si 2p)peaks for each sample scan. The ratio of the C—Si to the normalized areafor the Si 2p peak (C_(Si)/Si) was then compared between differentalkylated surfaces. Si NWs functionalized with methyl group were used asa reference surface for the other alkylated Si NW surfaces. The surfacecoverage for each alkyl group is thus reported as(C_(Si)/Si)_(alkyl)/(C_(Si)/Si)_(methyl). XPS survey spectra offreshly-prepared alkyl-functionalized Si NWs showed Si, C, O and Au withlittle (less than 1%) or no detectable Mg or Cl peaks. No oxide peaks inthe XPS spectra were observed on these freshly-prepared alkylatedsurfaces.

Occasionally, a small oxygen signal was observed at 532 eV (O 1s). Thissignal was assigned to adventitious adsorbed hydrocarbons having oxygenbonded to carbon (286 eV) as a result of wet chemical etching andsubsequent exposure to air. No SiO₂ was observed in the high resolutionSi 2p XPS scans as well as in energy dispersive spectrometry (EDS)measurements, further supporting the origin of the O 1s to be due toadventitious O on the surface. The lack of a fluorine 1 s (F 1s) signalin the XPS survey data, which would have appeared at 686 eV bindingenergy, confirmed that the NH₄F_((aq))-etched silicon surface was notfunctionalized with Si—F species.

The silicon nanowires surfaces of the present invention have beenfunctionalized with various saturated and unsaturated organic moleculeshaving single, double and triple bonds, by the two stepchlorination/alkylation reaction (Grignard reagent).

FIGS. 8A-8H show high resolution XPS scans of Si 2p and C 1 s regions ofpropyl, propenyl, propynyl and methyl-functionalized silicon nanowiressurfaces (CH₃—CH₂—CH₂—Si, CH₃—CH═CH—Si, CH₃—C≡C—Si, CH₃—Si,respectively). In Si 2p spectrum the ratios of Si 2p_(3/2) andSi2p_(1/2) produce the expected 2:1 area ratio having 0.6 eV energyseparation. No SiO₂ is detectable after alkylation (FIGS. 8A, 8C, 8E,and 8G). Three peaks were observed at 284±0.1%, 285.2 d 0.1% and286.6±0.1% eV in C 1 s spectrum (FIGS. 8B, 8D, 8F, and 8H). The peaks at285.2±0.1% and 286.6±0.1% eV are common to all alkylated andH-terminated Si surfaces, whereas the low binding energy peak at284±0.1% eV is unique to alkylated Si nanowire surfaces.

The ratio of C—Si/Si 2p peak area provides quantitative informationregarding the coverage of Si NWs with the various functional groups.Since methylation has been shown to provide a nearly complete monolayeron the Si (111) surface, Si—CH₃ has been used as a reference tocalculate coverage percentages of the other functional groups.Si—CH═CH—CH₃ surfaces showed a C—Si/Si 2p peak ratio indicating fullcoverage relative to that of CH₃—Si surface. Thus, Si—CH═CH—CH₃ can bepacked at very high density by using the two stepchlorination/alkylation method presented herein. Similarly, C—Si/Si 2ppeak ratio for Si—C≡C—CH₃ and Si—CH₂—CH₂—CH₃ produced coveragepercentages of 97±5% and 60±5%, respectively, in comparison to methylcoverage.

Example 5 Stability of the functionalized Si NWs

Functionalized Si NWs were exposed to ambient conditions for severalweeks, to assess their stability. The degree of oxidation was extractedfrom the ratio between the integrated area under the SiO₂ peak (103.5eV) and the Si 2p peak.

The stability of the wires decreased monotonically with the alkyl chainlength (Table 2; FIG. 9). Methyl (C₁)—functionalized Si NWs that wereexposed to air over a period of more than two weeks (336 hours) showedan increased stability (>1.5 times more) than equivalent 2D Si (100)surfaces. Moreover, all molecules showed stability of one monolayer(corresponds to SiO₂/Si_(2p) peak ratio of 0.19-0.26) after 46 dayswhereas in Si 2D (100) the stability of one monolayer (corresponds toSiO₂/Si_(2p) peak ratio of 0.15-0.18) was obtained after 8 days. Withoutbeing bound by any theory or mechanism of action, these differences instability could be attributed to the higher surface coverage ofmolecules on Si NWs, than on equivalent 2D Si (100) surfaces.

TABLE 2 Summary of the oxidation SiO₂/Si_(2p) ratio for alkylated Si NWsand 2D Si (100) surfaces at representative exposure times to ambientconditions Exposure time to air Molecule 0 hr 24 hr 48 hr 336 hr Methyl(C₁) - Si NW 0 0 0 0.04 Methyl (C₁) - 2D substrate 0 ND ND 0.11 Ethyl(C₂) - Si NW 0 0 0 0.03 Ethyl (C₂) - 2D substrate 0 0.03 0.08 0.13Propyl (C₃) - Si NW 0 0.01 0.07 0.13 Propyl (C₃) - 2D substrate 0 ND NDND Butyl (C₄) - Si NW 0 0.02 0.07 0.13 Butyl (C₄) - 2D substrate 0 ND NDND Pentyl (C₅) - Si NW 0 0.02 0.06 0.14 Pentyl (C₅) - 2D substrate 0 NDND ND Hexyl (C₆) - Si NW 0 0.01 0.06 0.12 Hexyl (C₆) - 2D substrate 00.04 0.08 0.18 ND = not determined

Example 6 Fabrication of the Si NW Field Effect Transistors

Devices were fabricated by depositing four Al electrodes on anindividual Si NW on top of a 90 nm thermally oxidized degenerately dopedp-type Si (0.001 Ω·cm⁻¹) substrate. The electrodes were mutuallyseparated by 1.70±0.05 μm (FIG. 10). For each Si NW field effecttransistor device, the intrinsic conductivity at determined back gatevoltage was obtained by the four-point probe method. Particularly,electrical properties collected with the four-point probe method enablethe configuration wherein there is no contact resistance between themetallic contacts and the Si NW.

Example 7 Sensing Characterization of Si NW Field Effect Transistors

The developed sensors were placed in a 316-stainless steel chamber withPTFE O-rings. To assess the sensing characteristics of the various SiNWs, current-voltage measurements at determined back gate voltage ofeach sensor were performed with digital multimeter (model 34411A;Agilent Technologies Ltd.) that is multiplexed with 40-channel armaturemultiplexer (model 34921A; Agilent Technologies Ltd.). In thesemeasurements, a voltage of −3 V was applied to the degeneratively dopedsilicon substrate that was coated with 200 nm aluminum, as an ohmiccontact. The −3 V back-gate-voltage value was chosen to provide anoptimal signal-to-noise ratio of the output signal. Under this value ofback gate voltage, four-point probe transport measurements were carriedout, at bias range between −5 and +5 V, in steps of 10 mV, with the twoinner electrodes serving as voltage probes and the two outer electrodesserving as current probes.

A Labview-controlled automated flow system delivered pulses of simulatedmixtures of biomarker vapors at a controlled biomarker vapor pressureoptimized to the detector surface area. Dry air was obtained from ahouse compressed air source, controlled with a 10 L/minute mass flowcontroller. In a typical experiment, signals of sensor array elementswere collected for 70 seconds of clean laboratory air, followed by 80seconds of analyte vapors in air, followed by another 70 secondsinterval of clean air to purge the system. Data analysis of the signalscollected from all the sensors in the array was performed using standardprincipal component analysis.

Example 8 Evaluation of Sensitivity of Si NW Field Effect Transistors

The Si NW FETs of the present invention have improved sensingcapabilities in comparison to equivalent SiO₂ coated Si NW field effecttransistors.

Si NW FET devices were fabricated by integrating an individual Si NWwith metallic electrodes that were separated by 500 nm, on top of a 30nm SiOx that covers degenerately doped Si substrate. FIG. 11 show graphsof the responses of the devices to hexane vapor exposure atconcentration of 40 ppb, when applying −3 V back gate voltage, expressedas ΔR/R_(b) (where R_(b) is the baseline resistance of the detector inthe absence of analyte, and ΔR is the baseline-corrected steady-stateresistance change upon exposure of the detector to analyte). The graphsdemonstrate the improvement of sensing capabilities of the Si NW FETupon, oxide removal and further functionalizing the Si core.

Exposure of Si NW FETs with and without oxide layer (at the interfacebetween the organic layer and Si core) to 40 ppb hexane vapor (as arepresentative analyte) showed that removing the oxide coating andfunctionalizing the Si NW core via Si—C bond gives improved, stable, andreproducible responses as well as a high signal-to-noise ratios (FIGS.11A and 11B). For example, exposure of SiO₂-coated Si NW FET to 40 ppbhexane showed almost no significant differences between the responsescollected in turn-on/shut-off (ON/OFF) exposure cycles (FIG. 11B; uppercurve). Self-assembly of CH₃ groups on the surface of SiO₂-coated Si NW,via Si—O—Si (silane) bond, showed no improvement in the response signals(FIG. 11B; middle curve), as compared to SiO₂-coated Si NW having no CH₃groups. Without being bound by any theory or mechanism of action, thisobservation might be attributed to an inferior adsorption of hexanemolecules in the CH₃ layer. Functionalization of SiO₂-coated Si NW withlonger alkyl chains, such as butyl functional group, showed only minorimprovement (FIG. 11B; lower curve). As can be observed from FIG. 11B,exposure of SiO₂-coated Si NWs functionalized with butyl chains to 40ppb hexane showed irreproducible response signals that are within thenoise background of the sensor.

In contrast, exposure of non-oxidized alkyl functionalized Si NW FETs tovapor of hexane provided improved, stable, and reproducible responses aswell as high signal-to-noise ratio (FIG. 11A). Of note is the lowerresponses of non-oxidized methyl functionalized (upper curve) Si NW FETin comparison to that of non-oxidized butyl-functionalized (lower curve)Si NW FET, under the same experimental conditions. This could possiblybe attributed to the inferior ability of CH₃ layer to adsorb vaporanalytes, as compared to (longer) butyl chains. Nonetheless, theresponses of non-oxidized methyl functionalized Si NW FETs weresignificantly better than those obtained for oxidized methylfunctionalized Si NW FETs.

It is thus evident that the Si NW FETs of the present invention provideenhanced responses in comparison to equivalent SiO₂ coated Si NW fieldeffect transistors. Furthermore, the functionalities present in theadsorptive phase play significant role in achieving improved detectionlevels. Other properties, and in particular the affinity of thefunctional groups towards analyte molecules, also play important role inproducing high sensing capabilities. It is noteworthy that the devicepresented herein responds at very high sensitivity to minute (40 ppb)quantities of analyte molecules.

Example 9 An Analysis of Complex Multi-Component (Bio) Chemical Media

An array of non-oxidized Si NW FETs, in which each device isfunctionalized with different organic molecules, namely2-(4-chlorobutyl)-1,3-dioxolane, 4-chlorobutyl chloroformate,4-chlorobutyl benzoate, 1-chloro-4-phenylbutane, 4-chlorobutyl acetate,4-chloro-1-butanol, and C₃-C₈ alkyl molecules, was prepared according tothe principles of the present invention. The array of sensors wasexposed to breath patterns simulating either “healthy” breath or“cancerous” patient's breath.

The experiments were performed at ambient conditions using saturatedwater vapor background flow in order to simulate the background watervapor content of human breath. The simulated “cancerous” breathcontained a mixture of 40 ppb isoperene, 37 ppb hexanal, 19 ppb styrene,15 ppb heptanal, 24 ppb 1,2,4-trimethyl benzene, and 22 ppb decane (Chenet al., Meas. Sci. Technol., 2005, 16, 1535-1546). The simulated“healthy” breath contained a mixture of 26 ppb isoperene, 20 ppbudecane, and 29 ppb decane (Chen et al., Meas. Sci. Technol., 2005, 16,1535-1546). Multiple exposures to each mixture were performed and datawas obtained for the array of sensors. Principal component analysis wasperformed for the obtained signals.

FIG. 12 shows a clear discrimination between “cancerous” (C) and“healthy” (H) breath samples using the sensor array of the presentinvention. Additionally, the discrimination between simulated “healthy”and “cancerous” breath patterns was further improved by increasing thediversity of functionalized, non-oxidized Si NW FETs in the array ofsensors. These observations indicate that the developed sensingtechnology has a high potential for diagnosis, detection, and screeningof cancer as well as other diseases via breath samples. The highdetection capabilities of the developed electronic nose device renderthis technology advantageous over the traditional GC-MS that is used inconjugation with a pre-concentration system for similar applications.The devices of the present invention are capable of detecting differentvolatile biomarkers from breath at ppb level of concentrations.

Example 10 Sensing of Polar and a-Polar Target Molecules

Exposure of non-oxidized butyl-functionalized. Si NW FETs to variousa-polar volatile organic compound targets showed correlation between thelength of target molecules and the functionality of the Si NW surface.A-polar target molecules (i.e., molecules having approximately zerodipole moment) which possess longer alkyl chains produced smallerelectrical responses in absolute value (hexane>heptane>octane). Withoutbeing bound by any theory or mechanism of action, the Si NW FETs havinga butyl functional group at the monolayer/air interface, provide theadsorption of a-polar target molecules preferably between the butylchains. The longer the alkyl chain, the lower the adsorption probabilitybetween molecular chains resulting in smaller responses (Table 3). Thus,it is presumable that the adsorption of a-polar targets between themolecular chains of the monolayer induces conformational changes in theorganic monolayer. These conformational changes affect either thedielectric constant and/or the effective dipole moment of the organicmonolayer, which, in turn, affects the conductivity pass through thenanowire.

TABLE 3 Response of non-oxidized butyl-functionalized Silicon NW FETs tovarious chemical vapors at 40 ppb. The dipole moment values of thedifferent analytes are indicated. Analyte Dipole [D] Response (ΔR/R_(b))Hexane 0 −0.043 ± 0.04 Heptane 0 −0.035 ± 0.04 Octane 0 −0.031 ± 0.03Trichloroethylene 0.80 −0.078 ± 0.04 Ethanol 1.69 −0.110 ± 0.05 Ethylacetate 1.78 −0.122 ± 0.04

The non-oxidized butyl-functionalized Si NW FETs of the presentinvention were exposed to various polar VOC targets, namelytrichloro-ethylene, ethanol, and ethyl acetate (Table 3). A correlationbetween the Si NW response and the dipole moment of the target moleculewas obtained. Target molecules having higher dipole moment valuesproduced higher sensor responses in absolute value. Without being boundby any theory or mechanism of action, the higher responses of polarmolecules, with respect to a-polar molecules indicate that the sensingprocess of polar molecules involves at least one additional mechanism.This observation might be attributed to either one of the followingscenarios. In the first scenario, the polar molecules adsorb on/in themonolayer close to the NW surface and induce direct electrostaticinteraction with the NW charge carriers. In the second scenario, thetarget molecules change the dielectric constant and/or effective dipolemoment of the organic monolayer, thus affecting the NW conductivity. Itis thus concluded that minute concentrations of polar VOCs are capableof producing large electronic responses of in non-oxidizedbutyl-functionalized Si NW FETs.

While the present invention has been particularly described, personsskilled in the art will appreciate that many variations andmodifications can be made. Therefore, the invention is not to beconstrued as restricted to the particularly described embodiments, andthe scope and concept of the invention will be more readily understoodby reference to the claims, which follow.

The invention claimed is:
 1. In an electronic device comprising an array of chemically sensitive sensors for detecting volatile organic compounds in a sample, the chemically sensitive sensors comprising field effect transistors comprising functionalized silicon nanowires, the improvement wherein: said field effect transistors are non-oxidized functionalized silicon nanowires comprising surface Si atoms and a plurality of functional groups, which form a direct Si—C bond with the silicon nanowires, wherein Si is a surface Si atom and C is a carbon atom of the functional group, wherein the plurality of functional groups are selected from the group consisting of propyl, pentyl, hexyl, octyl, decyl, and undecyl, and wherein coverage of the surface Si atoms by said plurality of functional groups is at least about 50%.
 2. In a system comprising an electronic device comprising an array of chemically sensitive sensors for detecting volatile organic compounds in a sample, the chemically sensitive sensors comprising field effect transistors comprising functionalized silicon nanowires, wherein said chemically sensitive sensors output sensor output signals, and a learning and pattern recognition analyzer that receives the sensor output signals from said electronic device and analyzes the signals to produce an output signature, the improvement wherein said electronic device is an electronic device in accordance with claim
 1. 3. The system according to claim 2, wherein the learning and pattern recognition analyzer comprises at least one algorithm selected from the group consisting of principal component analysis (PCA), artificial neural network algorithms, multi-layer perception (MLP), generalized regression neural network (GRNN), fuzzy inference systems (FIS), self-organizing map (SOM), radial bias function (RBF), genetic algorithms (GAS), neuro-fuzzy systems (NFS), adaptive resonance theory (ART), partial least squares (PLS), multiple linear regression (MLR), principal component regression (PCR), discriminant function analysis (DFA), linear discriminant analysis (LDA), cluster analysis, and nearest neighbor.
 4. The electronic device according to claim 1, wherein said chemically sensitive sensors output sensor output signals. 